The present invention relates to a system for preparing coating powder, particularly enamel powder, for spraying on various articles.
A powder preparation system which is part of a system for spraying enamel powder on articles is known from U.S. Pat. No. 4,500,560. In this known system, air is fed continuously through a perforated bottom of a powder preparation container to maintain the powder in a fluidized state. To moisten the enamel powder and to maintain the moisture at a given value, a vapor is fed into the container at a rate depending on the moisture content of the mixture of enamel powder and air in the container. The fluidized/moistened powder is then delivered to a spraying device by means of a Venturi based, gas operated, injector which is coupled to the interior of the container and designed to draw powder from the container. The suction effect of the injector is variable and controlled by the amount of air applied to the injector.
Another powder preparation system, for enamel or other coating powders, includes a preparation container for preparing fluidized powder and is described in Federal Republic of Germany Patent No. 36 02 388 (U.S. Pat. Application Ser. No. 006,952). In the container there is an elongated bell which is immersed into the fluidized powder with the open end of the bell facing down. As occurs with an upside down cup that is immersed in water, an air pocket is developed in the bell which prevents the fluidized powder from entering into the bell. Consequently, the height or level of fluidized powder within the bell is lower than outside the bell. Within the bell, several fluid inlets are disposed for the purpose of spraying water vapor or water into the fluidized powder to moisten the powder from within the bell.
Other powder coating systems using enamel powder are known from French Patent No. 1347012 which describes, for example, a mechanism for producing water vapor for moistening enamel powder. The vapor producing mechanism is disposed below the article to be coated while the article is sprayed with enamel powder from a spraying device. In another embodiment, water is mixed by means of spray nozzles into the spray jet of enamel powder which is directed at the article to be coated from a spraying device.
Enamel powder, like plastic and other powders that are suitable for forming a coating on an object, is electrostatically charged to cause the powder to be drawn to and remain adhered to an article on which it is sprayed. The article to be coated is normally connected to a ground potential so as to attract the powder which is charged to a high electrical potential.
Certain types of powders, particularly enamel powders, have the drawback that they can not be charged electrostatically to a degree that would cause the powder particles to arrive at and remain adhered to the article being sprayed. To overcome the problem, additional agents are typically admixed with the enamel. One such agent is silicone which allows enamel powder to be charged electrostatically more vigorously. To improve the quality of the coating on an article, enamel powder is additionally moistened with water or water vapor prior to being sprayed onto an article.
However, powder that is insufficiently moistened can not be charged effectively. As a result, the quality of the coating is poor and the coating process is inefficient because a large number of powder particles never reach or bounce off the article. On the other hand, a powder can not be overmoistened because overmoistened powder does not adhere well to an article to be coated. The key factor is the electrical conductivity of the powder which varies based on the moisture content of the powder. Accordingly, it is important to provide an ability to control the moisture content of the powder accurately and over a wide range of moisture values.